LED-based light source utilizing asymmetric conductors

Abstract

A light source includes LED dies that are flip-chip mounted on a flexible plastic substrate. The LED dies are attached to the substrate using an asymmetric conductor material with deformable conducting particles sandwiched between surface mount contacts on the LED dies and traces on the substrate. A diffusively reflective material containing light scattering particles is used instead of expensive reflective cups to reflect light upwards that is emitted sideways from the LED dies. The diffusively reflective material is dispensed over the top surface of the substrate and contacts the side surfaces of the dies. The light scattering particles are spheres of titanium dioxide suspended in silicone. The light source is manufactured in a reel-to-reel process in which the asymmetric conductor material and the diffusively reflective material are cured simultaneously. A silicone layer of molded lenses including phosphor particles is also added over the mounted LED dies in the reel-to-reel process.

Description

CROSS REFERENCE TO RELATED APPLICATION[0001]This application is a continuation-in-part of, and claims priority under 35 U.S.C. §120 from, nonprovisional U.S. patent application Ser. No. 12 / 941,799 entitled “LED-Based Light Source Utilizing Asymmetric Conductors,” by Yan Chai and Calvin B. Ward, filed on Nov. 8, 2010, the subject matter of which is incorporated herein by reference.TECHNICAL FIELD[0002]The present invention relates generally to packaging for light-emitting diodes, and more particularly, to a low-cost method of making an LED light source with improved light extraction characteristics.BACKGROUND INFORMATION[0003]Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy into light. Improvements in these devices have resulted in their use in lighting fixtures as replacements for conventional incandescent and fluorescent light sources. LEDs have significantly longer lifetimes than both incandescent bulbs and fluorescent tubes. ...

AI Technical Summary

Benefits of technology

This patent describes a way to use particles in a material to reflect light emitted from LED dies. This helps to direct the light away from the substrate, without needing to place each LED die in a separate reflective cup. This can save money and improve the performance of the LED devices.

Problems solved by technology

The packaging costs and reliability problems that result from having to use large numbers of individual LEDs present challenges that must be overcome if LED-based light sources are to reach their full potential as replacements for conventional light sources.

The wire bonds present problems in terms of fabrication cost and reliability, particularly when the light source includes a large number of individual dies.

The wire bonds are subject to failure both at the time of initial implementation of the bonds and later due to stresses between the phosphor layer and the encapsulated wire bonds.

In addition, the wire bonds block a significant fraction of the light leaving the LEDs, as both the bond pads and the gold wire absorb light.

However, this aspect requires a more complex mounting substrate having reflective cups.

The cost of the substrate increases the cost of the light source.

In addition, mechanical problems that arise from differences in the thermal coefficient of expansion between phosphor layer 37 and printed circuit board 32 become worse as the operating temperature increases.

While the arrangement shown in FIG. 2 reduces the problems associated with the wire bonds, heat dissipation and the loss of light that exits through the sidewalls of the LEDs remains problematic.

If the LEDs are mounted in reflective coos as described above, the cost of the substrate becomes a problem.

Furthermore, the bonding process requires that the LEDs be pressed against the printed circuit board during the bonding process, and hence providing a pressure mechanism that can operate on all of the LEDs in a light source at once is problematic if the LEDs are in reflective cups.

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